The adhesive bond between a pressure-sensitive adhesive tape and a substrate to which it has been applied tends to become gradually stronger so that after some hours, days or months, the tape can no longer be removed without delaminating either the tape or the substrate. Accordingly, peelable tapes such as masking tapes usually have very low initial adhesiveness and should be removed as soon as possible before any appreciable buildup in adhesion. Because of their low adhesiveness, such tapes often accidentally fall off.
It is believed that the highest quality pressure-sensitive adhesive tapes now on the market are those of U.S. Pat. No. Re 24,906 (Ulrich). Their adhesives are copolymers of about 88 to 97 parts of acrylic acid ester of non-tertiary alcohol, the alkyl groups of which have an average of 4-12 carbon atoms, and correspondingly about 12 to 3 parts by weight of at least one strongly polar copolymerizable monomer such as acrylic acid. Those adhesives experience no observable deterioration even after years in hot climates, but experience the adhesion buildup mentioned above, as is pointed out in U.S. Pat. No. 3,008,850 (Ulrich). That later Ulrich patent suggests that less buildup would occur if the modifying copolymerizable monomer were acrylonitrile or methacrylonitrile which are somewhat less polar. Even so, tapes of 3,008,850 experience gradually increasing adhesion to substrates, and there is no assurance that a tape which provides adequate initial adhesion will be removable without delamination after some months at ordinary room temperatures.
Recent patents disclose that pressure-sensitive adhesive tapes like those of the Ulrich patents can be made by photopolymerizing the same monomers in contact with a carrier web. See U.S. Pat. Nos. 4,181,752 (Martens et al.); 4,303,485 (Levens); 4,329,384 (Vesley et al.); and 4,330,590 (Vesley). All of the tapes of the working examples of those patents experience the same sort of adhesion buildup as do tapes of the Ulrich patents.